I should have done this BEFORE he went around meeting people at camp but. Uh. I'm totally not used to having inhuman characters who might register on people's power-o-meter!
So, on Vash and what he will look like to people who can see weird shit.
It's a little hard to explain what Vash is, partly because the series never fully explains it themselves. The nearest I can pin down is that he is raw, living, organic energy. PLANTs are used to power pretty much everything in human society at this point, and THEY look like glowing, ethereal angels who can grow and produce just about anything from their body.
What this means is while Vash clearly HAS a soul, I doubt it's significantly different from anyone else's soul, except maybe that it's really emo. His power is physical, not spiritual.
So what does this mean? Well. Up to you, I'm not going to tell anyone what they can or can't do, I assume you know your character well enough. People who can sense/see spirits might be able to tell that Vash isn't human, but I'm not sure if they'll be able to tell he's very powerful. And if there is anyone out there who can sense raw energy, Vash will probably set off all your radars.
If you are the sort that can SEE things... I dunno what you'll see. Vash doesn't have invisible wings or anything weird like that, and his power is contained within him unless he wants it to not be. Probably it would be similar to seeing a big glowy white overlay on Vash's body? I'm tempted to say it works like Chakra, but it doesn't flow through certain parts of his body, it's what he is.
As it is, do whatever you like as far as this goes. For pure curiosities sake I would like it if you told me here what your character will see or notice, if anything. But it's not a big deal and I won't keep tabs on it. Vash never outright SAYS what he is in canon, but it's cause he can usually avoid the subject easily. He doesn't make a particularly big deal to keep his 'secret' hush hush. And I think the only reason he keeps it quiet at all is because the humans of Gunsmoke are likely to be prejudice. One of Vash's earliest desires since childhood is to be accepted for what he is, so he won't mind being recognized as something different than human as long as you don't try to kill him for it (hi Ishida).